The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known hydraulic systems for vehicular transmissions include hydraulic pumps and fluidic circuits that provide pressurized fluidic flow to effect gear lubrication, transmission and motor cooling, and clutch activation. A hydraulic pump can be directly driven by an internal combustion engine with a rotational speed proportional to engine speed. Preferably, the hydraulic pump is configured to provide sufficient hydraulic pressure to the fluidic circuit to meet peak flow and pressure demands, such as hydraulic fluid demand during shifting of the transmission, hot engine idle speed garage shifts, and, in certain transmissions, a hydraulic park mode in an electronic transmission range selector system. Additionally, hydraulic pressure and flow requirements of the transmission may still exist when the engine (and thus the engine-driven pump) is off, such as to provide pressurized hydraulic fluid for clutch activation during operation in an electric-only operating mode in a hybrid electro-mechanical powertrain.
As engine speed increases, pump speed of an engine-driven hydraulic pump increases, thereby increasing hydraulic flow rate and increasing torque load used to drive the pump. This consumes engine power, negatively affecting fuel economy. Limiting the hydraulic flow rate decreases the torque load of the engine used for the engine-driven hydraulic pump. On-demand pumps including binary pumps are configured for full or a partial volumetric output. Operating such pumps at partial volumetric output decreases the torque load of the engine used by the pump thereby improving fuel efficiency. Fluidic flow and associated hydraulic line pressure are affected by pump speed and the temperature of the hydraulic fluid. Hydraulic pressure and flowrate through a fluidic circuit are interdependent. It is appreciated that fluid flow rate through a hydraulic circuit is proportional to the pressure difference through the flow path.
Physical packaging limitations place constraints on transmission designs, including location and size of a hydraulic pump. For example, an engine-driven, i.e., an on-axis hydraulic pump has a rotational axis that is aligned with an axis of rotation of an input member of the transmission. Therefore, an on-axis pump affects packaging, including extending an overall length of a transmission. An off-axis pump has an axis of rotation offset from and eccentric to the transmission axis, e.g., an electrically powered auxiliary hydraulic pump. Off-axis pumps offer increased flexibility in packaging location.
A hybrid powertrain can include a hydraulic system to provide pressurized hydraulic fluid for a number of functions throughout the powertrain. These functions include low pressure high volumetric fluidic flow for cooling for electric motors, and high pressure, low volumetric fluidic flow for clutch activation to effect torque transfer between input and output members associated with the engine, electrical machines, and driveline. Various control schemes and operational connections between the aforementioned components of the hybrid drive system are known, and the control system must be able to engage and disengage the various components from the transmission in order to perform the functions of the hybrid powertrain system. Hydraulic control systems can be used to lubricate mechanical devices such as bearings and planetary gear sets.
Pumps provide pressurize hydraulic fluid within a hydraulic control system for a transmission. In addition to a mechanically-driven hydraulic pump coupled to an engine output member, a hydraulic control system for a transmission may include an electrically-driven auxiliary pump to supplant the mechanically-driven hydraulic pump. Known hybrid powertrain systems operate with the engine in engine-on and engine-off operating states. When operating a hybrid powertrain system in the engine-off state, the mechanically driven hydraulic pump cannot provide a supply of pressurized hydraulic flow to the hydraulic control system. Instead, the electrically-driven auxiliary pump provides hydraulic line pressure required to operate the powertrain system.
The main on-axis hydraulic pump supplies hydraulic line pressure necessary for sufficient hydraulic flow for the transmission and all components of the hybrid drive system at engine idle speeds. Therefore, at operating speeds greater than engine idle the main on-axis hydraulic pump produces excess hydraulic line pressure thereby producing an excess hydraulic flow.